In recent times a tendency has grown to obtain a variety of chemicals from renewable resources. In this context there has been a tendency to create chemicals from biomass carbohydrates, such as cellulose, starch, hemicellulose, sugars and the like. Under dehydration conditions these carbohydrates can be converted into a number of interesting chemicals, including levulinic acid and derivatives and hydroxymethylfurfural and derivatives. It would be of interest to use these chemicals for the production of added value chemical compounds.
One such added value chemical compound is constituted by terephthalic acid. Terephthalic acid is widely used in the preparation of polyesters. To make these polyesters the terephthalic acid is usually reacted with a diol, such as ethylene glycol, trimethylene glycol butane diol, or higher glycols. The polyester thus obtained finds ample application in containers, bottles, films, fibres and other packaging materials. Terephthalic acid is commonly prepared by the oxidation of para-xylene. Para-xylene is generally obtained from a refinery process in the treatment of crude oil, e.g. from a reforming process. It is evident that crude oil is not a renewable resource. Therefore, it would be desirable to provide a process wherein terephthalic acid would become available from a renewable source, such as biomass carbohydrates. Also para-xylene production from crude oil is tight and may hamper future PET growth, specifically in Asia.
In U.S. Pat. No. 7,385,081 a process has been disclosed wherein biomass is being dehydrated to yield 5-hydroxymethylfurfural, which can be oxidized to yield 2,5-furandicarboxylic acid. The dicarboxylic acid obtained is subsequently subjected to a Diels Alder reaction with ethylene to yield a bicyclic ether which is then dehydrated to produce terephthalic acid. The process described in U.S. Pat. No. 7,385,081 indeed used renewable starting material. The process also showed that terephthalic acid can be obtained via this process. The conditions that can be applied to the Diels Alder reactions include an ethylene pressure in the range of from 10 psig to about 2000 psig (about 1.7 to 139 bar abs), with a preferred pressure of 100 to 300 psig (about 7.9 to 21.7 bar abs). Reaction temperatures may be varied between 100 and 250° C. The reaction may be conducted in the presence of a solvent, such as water, DMSO, N-methyl-2-pyrrolidone, N,N-dimethylformamide, C1-C10 alcohols, C2-C6 ketones and C2 to C10 esters. However, the examples in this patent specification wherein ethylene pressures ranging from 100 to 250 psig (7.9 to 18.2 bar abs) and temperatures of 100 to 200° C. were applied and water or methanol was present as solvent, show that the yield of terephthalic acid is very low. The highest estimated yield was 0.14 mole % based on the amount of 2,5-furandicarboxylic acid charged. Such low yields of the product following the Diels Alder reaction would render this process unsatisfactory and commercially unattractive.
Another route to arrive at terephthalic acid has been described in WO 2010/151346. According to the process described herein a hexose is converted to 5-hydroxymethylfurfural, which is hydrogenated to 2,5-dimethylfuran. This product is reacted with ethylene under cycloaddition reaction conditions and in the presence of a solid catalyst to produce p-xylene. The p-xylene, thus produced, may be oxidized to produce terephthalic acid. The cycloaddition reaction conditions typically include a temperature range of 100 to 300° C. and an ethylene partial pressure of about 10 to about 100 bar. The use of solvents, such as DMSO, is advantageously reduced or even eliminated so that solvent-free or substantially solvent-free reaction mixtures are employed. Amongst the solid catalysts mentioned reference is made to activated carbon, silica gel, alumina, zirconia and molecular sieves. These catalysts are present in small amounts such as 2% wt, based on 2,5-dimethylfuran. In the process byproducts are produced, such as 2,5-hexanedione. Although yields of the desired benzene derivative are higher than in the process of U.S. Pat. No. 7,385,081 the contamination with significant amounts of byproduct makes this process also unattractive.